Add storage backed token source

[andrewhowdencom]

In order to have a client application persist through multiple invocations (essentially the requirement for a CLI application), the token needs to be stored outside the application in some persistence. This commit provides that persistence.

The general design of this work is to wrap a token source, and if underlying token source changes, then write that token back to storage. Then, whenever the application is reloaded, refresh the token source being used with whatever was in the cache.

This also means that there needs to be something that "seeds" the cache; for example, if the user doesn't actually have authentication just yet. To address this, a new primitive called a "Seed function" (essentially the actual authenticatino process rather than refresh) was created. This allows plugging in the auth to the token refresh cycle.

== Design Notes
=== Storage Implementations

The primary storage implementation is a file, intended to be used with a path supplied by the XDG library. This would allow storing tokens in ~/.local/share/x40/.token.json, which is a logical place for them.

There's also an implementation in memory (called "Test"), designed for ... well, testing. There's not really a limit to what other interfaces can be provided in future — I'd probably try and write a BoltDB one if this filesystem one proved buggy (so as to defer the hard™ parts to boltdb).

=== NewCacheSource

The signature for new cache source is complicated. The problem is testability — the underlying oauth library has a bunch of stuff private, which prevents assertions about its state or reuse within tests.